Tension control for strip mills



1939 A, D. MOCREARY 2,168,777

TENSION CONTROL FOR STRIP MILLS Filed May 5, 1937 Alfred D. MCC/Guwy ratemecl Aug. 8, 1939 UNITED STATES PATENT OFFICE TENSION CONTROL FOR STRIP MILLS Application May 5, 1937, Serial No. 140,800

3 Claims.

This invention relates to the art of rolling and, in particular, to the rolling of strip.

It is customary in the rolling art to cold roll steel strip by passing it back and forth through a reversing mill, winding and unwinding the strip in coils as it is rolled by means of suitable reels. It has also been proposed to connect a dynamoelectrlc machine to each reel, driving the machine connected to the winding reel as a motor and operating the machine connected to the unwinding reel as a generator to apply back tension to the strip entering the mill. Current regulators associated with the dynamo-electric machines cause them to be traversed by a constant current to maintain the strip leaving and entering the mill under a constant tension at all times. This arrangement is very satisfactory for so long as the mill is stationary or is running at normal speed. When the mill is started or stopped, however, because the mass of the parts to be accelerated or decelerated is considerable, wide variations are introduced into the tension applied to the strip with the possibility of breakage of the latter, making it necessary to exercise great caution during the starting and stopping periods and effect the speed changes quite slowly. This reduces the tonnage output per day and is further objectionable because variations in the tension cause changes in the percent reduction effected in the strip and so give rise to gauge variations which are highly undesirable.

I have invented a novel system for controlling the tension applied to the strip entering and leaving the mill so as to avoid variations therein when startingor stopping. In accordance with the invention, I provide means for modifying the action of the current regulators in accordance with the changes in the speed of the strip or the peripheral speed of the mill. I preferably employ pilot generators driven at speeds proportional to those of the strip entering and leaving the mill and connect the primary windings of suitable transformers to the terminals of the generators. The secondary windings of the transformers are connected to the current regulators in such manner as to modify their usual action. Qurrent flows in the secondary windings of the transformcrs only when the speed of the strip or mill is changed and during normal operations the regulators perform in the ordinary manner to maintain constant tension on the material. A preferred embodiment of the invention is illustrated in the accompanying drawing which is a schematic illustration of a reversing strip mill and sufllcient of the associated driving and control apparatus for an explanation of the invention.

Referring in detail to the drawing, a reversing mill it comprises work rolls II and backing rolls 1?. A motor l3 drives the mill rolls through any suitable connection. A generator l4 driven by a suitable type of motor or prime mover supplies current to the motor it. The field winding of the generator !4 is indicated at [5 and is com trolled by a reversing switch I8 and the usual field rheostat (not shown). The field reversing switch has four contacts, lG-i, i6-2, l63 and lS- l. The first and last of these are shown closed and it will be assumed that the polarity of the generator 14 induced thereby is such as to cause the motor l3 to drive the mill rolls and feed the strip in the direction indicated by the arrows. The excitation circuit of the motor I3 is not shown but is the same as usually employed. Reels ii and i8 serve alternately to unwind a coil, to feed strip through the mill, and rewind it into a coil on the exit side. Dynamo-electric machines l9 and 20, which may conveniently be ordinary direct current machines of the shunt type, are connected to the reels ll and 58. The field windings of the machines are shown at 25 and 22, each being provided. with a rhecstat for controlling the excitation in accordance with the diameters of the coils on the reels. When strip is passing through the mill in the direction indicated by the arrow, the machine it! operates as a motor while the machine 69 is driven as a generator. The current for the motor 20 is supplied by a generator 2Q. 'lfhe circuit connec ing the generator 2% to the motor 28 includes con= tactors 25 and 26 and a series resistor 21'. The field winding of the generator lid is shown at 28 and its energization is controlled by a regulator 29. The regulator 2% has a main coil 36 and an armature 32, a fixed contact and an auxiliary coil 33. The relay illustrated is of the vibrating armature type, although other types of relays may be substituted.

The circuit for the field winding 28 extends from the positive exciter bus through a resistor 34 or the armature 3i and contact 32 of the regulator if they are in engagement, and through a contactor 35 which is closed when rolling in the direction indicated by the arrows which will he considered the forward direction. The coil 30 of the regulator is connected across the series resistor-2l, in series with an adjusting rheostat 36. This rheostat is adjusted so that when the current supplied to the motor 20 exceeds a certain Value, which is so chosen as to cause the motor to exert suflicient torque on reel iii to provide the desired forward tension in the strip leaving the mill, the voltage impressed on the coil 30 which is the I. R. drop across the resistor 21, energizes the coil sufiiciently to raise the armature 3|, thereby opening the shunt established around the resistor 34 by the engagement of the armature with its contact. The insertion of the resistor 34 in series with the field winding 28 of the generator 24 reduces the excitation and, therefore, the terminal voltage of the generator 24 and the current supplied thereby to the motor 20. Should this current decrease below a predetermined value, the reverse of the above cycle occurs and the resistor 34 is shunted to cause the generator 24 to supply more current to the motor 213.

The dynamo-electric machine i3, operating as a generator when the strip is moving forward, is connected in parallel with a generator 24 by a contactor 31. A booster generator 38 is connected in series with the machine I9 so that the terminal voltage of the combination will substantially equal that of the generator 24. The generator 38 has a field winding 39, the excitation of which is controlled by a regulator 48 similar in all respects to that shown at 29, to which the field winding is connected by a contactor 4|. The main coil of the relay 40 is indicated at 42, the auxiliary coil at 43, the armature at 44, the fixed contact at 45, and the control resistor at 46. The adjusting rheostat 4? is connected in series with the coil 42 across a series resistor 43 in the circuit between the machine l9 and the generator 38. If the current supplied by the machine l9 operating as a generator, and the booster generator 38 to the motor 20 is above a predetermined value, the relay 40 operates to remove the shunt around the resistor 46, thereby decreasing the excitation of the generator 38. This lowers the terminal voltage of the combination of machine i9 and generator 38 and causes them to supply less current to the motor 20. If too little current is supplied, the reverse takes place, raising the terminal voltage of. the generator 38 and causing a greater portion of the current for the motor 20 to be supplied by the machine 19.

Pilot generators 49 and 50 are arranged in any convenient manner so as to be driven at speeds proportional to those at which the strip enters and leaves the mill. As shown,- the pilot generators are directly connected to the shafts of the machine l9 and 20 in the manner of exciter generators. The generators 49 and 5B are connected to the primary windings of transformers 5| and 52. The secondary windings of these transformers are connected to the coils 43 and 33 of the regulators 43 and 29 respectively. The generators 49 and 50 are direct current machines. When operating at normal speed, therefore, they impress a certain voltage across the primary windings of the transformers, but since this is constant and the current is unidirectional, there is no change in the flux induced in the transformer cores and consequently no voltage is induced in the secondary windings. When the mill is starting or slowing down to a stop, the current through the primary windings of the transformer varies until steady conditions are obtained. The resulting change in the flux threading the cores of the transformers induces a voltage across the terminals of the secondary windings and causes a flow of current through the auxiliary regulator coils 43 and 33. Connections are so made between the pilot generators, transformers and auxiliary regulator coils that, when the mill is being started, the coil 43 is cumulative with respect to the coil 42, to reduce the current traversing the machine I9. This avoids excessive tension which would otherwise be applied to the strip between the reel ll and the mill, because of the necessity for overcoming the inertia of the reel and the machine l9, as well as the resistance to rotation thereof resulting from flow of current through the machine is. The regulator coil 33 is so connected that it is differential relative to the coil 3!! and thereby permits a greater current than normal to flow through the motor 20. The motor 20 thus exerts a torque greater than nor mal to overcome inertia of the parts, and, at the same time, maintain the desired forward tension on the strip leaving the mill.

When the mill reaches normal speed, the regulator coils 43 and 33 are de-energized and a constant current is maintained through the machine l9 and the motor 20 to keep the strip on opposite sides of the mill under the desired tension. When the mill is slowed down preparatory to stopping, the polarity of the regulator coils 43 and 33 is reversed from that above described, because the flux in the transformer cores is decreasing instead of increasing. The coil 43 is thus differential relative to the coil 42 to increase the current traversing machine it. The desired tension on strip entering the mill is thus maintained and the inertia of the rotating parts overcome. At the same time, the coil 33 is cumulative relative 'to the coil 38 and decreases the current supplied to the motor 24 so that the inertia of the rotating parts which must be overcome does not serve to increase the tension in the strip leaving the mill.

When it is desired to reverse the mill, the reversing switch 96 is operated to close contactors iB-2 and l6-3 after opening the remaining contactors. At the same time, contactors 25, 35 and 4| are opened and corresponding contactors, 53, 54 and 55 are closed. These contactors may be controlled by any suitable means (not shown). The conditions of the dynamo-electric machines I9 and 20 and the regulators 40 and 29 are thus reversed from those already described but the cycle of operations is identical, the auxiliary coils of the regulators serving automatically to modify the normal action thereof during the starting and stopping periods to avoid the excessive or insufficient strip tension which would otherwise exist.

Since the invention overcomes the variation in strip tension occurring under the former practice, it makes possible the production of strip of more uniform gauge, as the tension on the strip has a direct bearing on the reduction effected. The danger of breakage of the strip because of excessive tension is avoided and the necessity for extreme care in starting -or stopping the mill is somewhat relaxed so that reversals can be made more speedily, thus increasing the production of the mill in a given time. The output of the generators 49 and 50 may be amplified, if desired, to obtain a more pronounced effect on the regulators.

Although I have illustrated and described herein but a preferred embodiment and practice of the invention, it will be understood that changes in either may be made without departing from the spirit of the invention or the scope of the appended claims.

I claim:

The combination with a rolling mill having winding and unwinding reels on opposite sides thereof, and dynamo-electric machines connected to said reels operating as a motor and generator respectively, of means for supplying current to said motor, regulators controlling the current through the machines, and means for modifying the action of the regulators mill speed, including pilot generators driven by said motor and generator, and transformers having their primary windings connected to said pilot generators.

2. The combination with a reversing mill including nected thereto,

sponsive directly to a change in the mill speed for modifying the action of said regulators, said means including direct current generators driven at speeds proportional to those of said machines, and transformers having their primary windings connected to said generators.

3. The combination with a strip reel having a dynamo-electric machine connected thereto, of a the current through the machine, a pilot generator driven at a speed prosaid reel, and a transformer ALFRED D. MCCREARY. 

